Impact of microstructure in uranium dioxide (UO2) on ballistic and electronic damage

During reactor irradiation, fuel pellets undergo a partial evolution of their microstructure. At high levels of burnup, a subdivision of grains into smaller grains in the peripheral areas of the fuel pellets - called high burn-up structure (HBS) - is observed. Similar changes also occur in the central regions of the pellets at elevated temperatures. These evolutions result from the combination of several factors, including the loss of energy from fission products. The effect of this damage could vary depending on the crystal orientation and grain size.
The main objective is therefore to understand how crystal orientation and grain size influence the damage caused by irradiation. Ion irradiation experiments will be conducted on single- and poly-crystalline UO2 samples at the JANNUS Saclay facility. In situ and ex situ characterizations using Raman and Rutherford backscattering (RBS-C) spectroscopy, transmission and scanning electron microscopy with Electron backscatter diffraction (EBSD) will be carried out.

Radiolysis and irradiation: coupled effects on the corrosion and hydrogen uptake kinetics of zirconium alloys in primary water of pressurised water nuclear reactors.

The materials that make up nuclear fuel assemblies, including zirconium alloys, are exposed to the reactor's primary environment, pressurised water at high temperature (around 150 bar, 300°C), neutron bombardment and ionising radiation (gamma, alpha in particular). The combination of these external factors leads to the degradation of materials through corrosion and the formation of irradiation defects. The latter have been shown to have a significant effect on the corrosion and hydrogen absorption kinetics of zirconium alloys. However, the impact of irradiation is material-dependent. The corrosion rate of Zircaloy-4 increases after irradiation of the metal and/or oxide, while the corrosion resistance of the M5 alloy is significantly improved under irradiation. The presence of niobium in the alloy undeniably plays a positive role under irradiation. However, its effect and the associated mechanism remain to be elucidated. Given the short lifetime of the transient radiolytic species and the interplay of the various phenomena mentioned above, in situ measurements are becoming mandatory. The PhD student will work on using and upgrading a unique existing device, co-developed with Framatome, to monitor online the effects of water radiolysis and irradiation on the corrosion and hydrogenation kinetics of various zirconium alloys in contact with a medium representative of the primary medium of pressurised water nuclear reactors. Understanding the mechanisms involved will enable a model to be developed and the first-order parameters to be determined. The thesis work will be promoted through publications and participation in - national and internation - conférences.

Operando Bragg coherent diffraction imaging to probe CO2 Reduction

The imperative to capture and convert CO2 into high value-added chemicals or fuels represents one of the most significant challenges in achieving a sustainable society. This reaction can be performed in the gas phase at high temperature but also electrochemically, at low temperature, not only mitigating the greenhouse effect, but also providing a way to store energy by transforming intermittent renewable electricity into high added value chemicals. This project aims to investigate the structural evolution of individual nanocrystals during CO2 reduction reactions. Using the unique capabilities of Bragg coherent X-ray imaging, we can dynamically map, in situ and operando, the three-dimensional changes in lattice deformation, strain, composition, and crystallographic defects of nano-crystallites, establishing a comprehensive experimental framework for structure-chemistry-performance relationships. The experiments will be conducted at ESRF, the European synchrotron facility located in Grenoble, in close proximity to CEA-Grenoble, within a leading international scientific environment. The project will be in collaboration with LEPMI (Laboratory of Electrochemistry and Physico-chemistry of Materials and Interface, Grenoble-France), which has expertise in electrocatalysis, materials science, and energy storage and conversion systems.

2D materials under irradiation for tomorrow's functionalities

In view of the challenges posed by global warming, some fundamental research is focusing on optimizing the properties of materials for gas capture (e.g. CO2), filtration, desalination or the conversion of water to H2 by photocatalysis. Two-dimensional materials (graphene, MoS2, hBN, etc.) nanostructured by ion irradiation have recently shown unique and original properties to improve the efficiency of these processes. The introduction of surface modifications to these materials can be used to tailor their properties to specific requirements. Irradiation by fast heavy ions, such as those produced on the GANIL facility, or by low-energy ions produced on CIMAP's PELIICAEN device, induces surface modifications on the nanometric scale.

In this thesis, we propose to gain a better understanding of the processes involved in ion-beam structuration and the modification of 2D material properties as a function of the influence of different irradiation parameters on the local radiation-induced modifications.

Catalysis using sustainaBle hOllow nanoreacTors wiTh radiaL pErmanent polarization

The combined demands of increasing energy production and the need to reduce fossil fuels to limit global warming have paved the way for an urgent need for clean energy harvesting technologies. One interesting solution is to use solar energy to produce fuels. Thus, low-cost materials such as semiconductors have been intensively studied for photocatalytic reactions. Among them, 1D nanostructures hold promise due to their interesting properties (high specific and accessible surfaces, confined environments, better charge separation). Imogolite, a natural hollow nanotube clay belongs to this category. Although it is not directly photoactive in the visible light range (high band gap), it exhibits a permanent wall polarization due to its intrinsic curvature. This property makes it a potentially useful co-photocatalyst for charge separation. Moreover, this nanotube belongs to a family sharing the same local structure with different curved morphologies (nanosphere and nanotile). In addition, several modifications of these materials are possible (wall doping with metals, coupling with metal nanoparticles, functionalization of the internal cavity) allowing tuning band gap. The proof of concept (i.e., photocatalytic nanoreactor) was only obtained for the nanotube form.

This phD project aims to study the whole family (nanotube, nanosphere, and nanotile, with various functionalizations) as nanoreactors for reduction reactions of protons and CO2 triggered under irradiation.

Quantum fragmented states in frustrated magnets

The last few decades of condensed matter research have seen the emergence of a rich new physics, based on the notion of "spin liquids". Interest in these new states of matter stems from the fact that they exhibit large-scale quantum entanglement, a property that is fundamental to quantum computation. By directly exploiting this notion of entanglement, a quantum computer would enable revolutionary approaches to certain classes of problems, compared with conventional computers.

The study of spin liquids is therefore a key technological issue, and the aim of this thesis project is to contribute to this fundamental research effort.

Exploration of the energy deposition dynamic on short time scale with laser-driven electron accelerator in the context of the Flash effect in radiotherapy

The objective of the thesis project is to analyze the physicochemical processes resulting from the extreme dose rates that can now be obtained in water with the ultra-short (fs) pulses of relativistic electrons produced by laser-plasma acceleration. Indeed, first measurements show that these processes are probably not equivalent to those obtained with longer pulses (µs) in the FLASH effect used in radiotherapy. To achieve this, we propose to analyze the dynamics of formation/recombination of the hydrated electron, an emblematic species of water radiolysis, to qualify and quantify the dose rate effect over increasingly shorter times. This will be done in three stages in support of the necessary and now accessible technological progress, to have a dose per pulse sufficient to directly detect the hydrated electron. First, with the existing UHI100 facility, using the scavenging of the hydrated electron by producing a stable species; then producing a less stable but detectable species in real time and increasing the repetition rate of the electron source. Finally, by using an innovative concept named a “hybrid target”, based on a plasma mirror as an electron injector coupled to a laser-plasma accelerator, delivering larger doses with a narrower energy spectrum, we will be able to develop pump-probe detection allowing access to the shortest times, and to the formation in clusters of ionization, of the hydrated electron and measuring its initial yield.

In situ and real time characterization of nanomaterials by plasma spectroscopy

The objective of this Phd is to develop an experimental device to perform in situ and real time elemental analysis of nanoparticles during their synthesis (by laser pyrolysis or flame spray pyrolysis). Laser-Induced Breakdown Spectroscopy (LIBS) will be used to identify the different elements present and to determine their stoichiometry.

Preliminary experiments conducted at LEDNA have shown the feasibility of such a project and in particular the acquisition of a LIBS spectrum of a single nanoparticle. Nevertheless, the experimental device must be developed and improved in order to obtain a better signal to noise ratio, to decrease the detection limit, to take into account the different effects on the spectrum (effect of nanoparticle size, complex composition or structure), to automatically identify and quantify the elements present.

In parallel, other information can be sought (via other optical techniques) such as the density of nanoparticles, the size or shape distribution.

Attosecond high reprate spectroscopy of ultrafast photoemission of gases

Summary :
The student will develop attosecond spectroscopy techniques making use of the new high reprate Ytterbium laser sources. The ultrafast photoemission dynamics will be studied to reveal in real time the processes of electron scattering/rearrangement as well as electron-ion quantum entanglement, using the charged-particle coincidence techniques.

Detailed summary :
In recent years, there has been spectacular progress in the generation of attosecond (1 as=10-18 s) pulses, rewarded by the 2023 Nobel Prize [1]. These ultrashort pulses are generated from the strong nonlinear interaction of short intense laser pulses with gas jets [2]. A new laser technology based on Ytterbium is emerging, with stability 5 times higher and reprate 10 times higher than the current Titanium:Sapphire technology. These new capabilities represent a revolution for the field.
This opens new prospects for the exploration of matter at the electron intrinsic timescale. Attosecond spectroscopy thus allows studying in real time the quantum process of photoemission, shooting the 3D movie of electronic wavepacket ejection [3,4], and studying quantum decoherence resulting from, e.g., electron-ion entanglement [5].
The first objective of the thesis work is to develop on the ATTOLab laser platform the attosecond spectroscopies using the new Ytterbium laser sources. The second objective is to take advantage of charged particle coincidence techniques, enabled by the high reprate, to study the dynamics of photoemission and quantum entanglement with unprecedented precision.
The student will be trained in ultrafast optics, atomic and molecular physics, quantum optics, and will acquire a broad mastery of XUV and charged-particle spectroscopy techniques.

References :
[2] Y. Mairesse, et al., Science 302, 1540 (2003)
[3] V. Gruson, et al., Science 354, 734 (2016)
[4] A. Autuori, et al., Science Advances 8, eabl7594 (2022)
[5] C. Bourassin-Bouchet, et al., Phys. Rev. X 10, 031048 (2020)

Towards multi-physics and multi-scale modelling of pilot-scale photo-electrochemical cells for hydrogen production

The production of chemical molecules and synthetic fuel, from non-fossil resources and renewable energy, is one of the solution envisaged to face climate issues. In this context, the use of photo-electrochemical cells (PEC) solar-driven water splitting is seen as promising route for hydrogen production. Today, proofs of concept generally concern small objects (of the order of 1cm² of active surface) and operating-times limited to a few minutes or a few hours. It is therefore essential, in order to consider the rapid deployment of PECs, to be able to predict the influence of the architecture of the cell and scale-up on their performance, in terms of energy efficiency, kinetic efficiencies (volume and surface ), stability of operation and aging of materials.
The thesis is part of the development of a generic simulation tool for PECs, in support of R&D. It will be carried out in collaboration with ENGIE LabCRIGEN (CIFRE funding), )Institut Pascal (host laboratory) and CEA (ISEC, IRIG and INES).
You have a solid background in Chemical Engineering, Energy, Fluid Mechanics or Applied Mathematics, with particular attraction for modelling and simulation; you have also a strong capacity for collaborative work, and you want to contribute actively the energy transition? By choosing this thesis, you will join a multidisciplinary consortium and contribute to an active field of research, at the interface between fundamental research and industry.